ios - 如何在 SwiftUI 环境中实现子上下文(CoreData)？

Question

我目前正在学习 SwiftUI，以进一步将其包含在我的日常基于 UIKit 的工作流程中。我已经到了无法在 SwiftUI 中运行一个概念的地步，我已经在 UIKit 中使用了多年。

这个想法是使用我的主要 CoreData managedObjectContext 的子上下文用于编辑实体。我基本上通过执行以下操作来实现它:

// Get view context of application
let viewContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

// Create NSManagedObjectContext
let editingContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)

// Set the parent of the editing context to the main view context
editingContext.parent = viewContext

通过使用单独的 editingContext我可以对其中的实体进行更改，而无需直接对我的主要上下文进行更改。如果用户选择中止更改，我只需重置 editingContext .

对于 SwiftUI 中的实现，我选择通过创建自定义 EnvironmentKey 将 editorContext 实现为环境对象。 :

struct EditingContextKey: EnvironmentKey {
    static let defaultValue: NSManagedObjectContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
}

extension EnvironmentValues {
    var editingContext: NSManagedObjectContext {
        get {
            return self[EditingContextKey.self]
        }
        set {
            self[EditingContextKey.self] = newValue
        }
    }
}

然后 editingContext被添加到我的 Root View 中:

let contentView = ContentView()
    .environment(\.managedObjectContext, viewContext)
    .environment(\.editingContext, editingContext)

至此，一切正常。我也可以使用 editingContext通过在各自的 View 中调用以下命令。

@Environment(\.editingContext) var editingContext

但是，一旦我对 editingContext 中的实体进行了更改，并尝试保存 editingContext ，我收到一个意外错误:

Fatal error: Unresolved error Error Domain=Foundation._GenericObjCError Code=0 "(null)", [:]: file

我已经检查过了，如果编辑实体的上下文匹配 editingContext .情况似乎如此。我有感觉，它可能与 SwiftUI 中的绑定(bind)有关，但不知道如何找到解决方案。

有没有人遇到过类似的问题？或者我对所需功能的基本方法是错误的，有更方便的方法吗？

谢谢!

Answer 1

我正在使用这样的子上下文:

struct ItemEditorConfig: Identifiable {
    let id = UUID()
    let context: NSManagedObjectContext
    let item: Item
    
    init(viewContext: NSManagedObjectContext, objectID: NSManagedObjectID) {
        // create the scratch pad context
        context = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
        context.parent = viewContext
        // load the item into the scratch pad
        item = context.object(with: objectID) as! Item
    }
}

struct ItemEditor: View {
    @ObservedObject var item: Item // this is the scratch pad item
    @Environment(\.managedObjectContext) private var context
    @Environment(\.dismiss) private var dismiss // causes body to run
    let onSave: () -> Void
    @State var errorMessage: String?
    
    var body: some View {
        NavigationView {
            Form {
                Text(item.timestamp!, formatter: itemFormatter)
                if let errorMessage = errorMessage {
                    Text(errorMessage)
                }
                Button("Update Time") {
                    item.timestamp = Date()
                }
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarLeading) {
                    Button("Cancel") {
                        dismiss()
                    }
                }
                ToolbarItem(placement: .navigationBarTrailing) {
                    Button("Save") {
                        // first save the scratch pad context then call the handler which will save the view context.
                        do {
                            try context.save()
                            errorMessage = nil
                            onSave()
                        } catch {
                            let nsError = error as NSError
                            errorMessage  = "Unresolved error \(nsError), \(nsError.userInfo)"
                        }
                    }
                }
            }
        }
    }
}

struct EditItemButton: View {
    let itemObjectID: NSManagedObjectID
    @Environment(\.managedObjectContext) private var viewContext
    @State var itemEditorConfig: ItemEditorConfig?
    
    var body: some View {
        Button(action: edit) {
            Text("Edit")
        }
        .sheet(item: $itemEditorConfig, onDismiss: didDismiss) { config in
            ItemEditor(item: config.item) {
                do {
                    try viewContext.save()
                } catch {
                    // Replace this implementation with code to handle the error appropriately.
                    // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                    let nsError = error as NSError
                    fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
                }
                itemEditorConfig = nil // dismiss the sheet
            }
            .environment(\.managedObjectContext, config.context)
        }
    }
    
    func edit() {
        itemEditorConfig = ItemEditorConfig(viewContext: viewContext, objectID: itemObjectID)
    }
    
    func didDismiss() {
        // Handle the dismissing action.
    }
}

struct DetailView: View {
    @ObservedObject var item: Item
    
    var body: some View {
        Text("Item at \(item.timestamp!, formatter: itemFormatter)")
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditItemButton(itemObjectID: item.objectID)
                }
            }
    }
}